1. Field of the Invention
This invention relates to a phonograph record player or changer, and more particularly to a combination of turntable drive and bearing arrangement.
Phonograph turntables referred to below are suitable for record players, whether manual or automatic, and automatic changers; as used herein, "record player" should be understood to include record changers.
When using highly sensitive stereophonic pick-up elements in phonographs, each movement of a record turntable, either in a plane parallel to the record or perpendicular thereto, will produce a signal in one or two channels of the stereophonic audio system. In a high fidelity phonograph the turntable is required to be capable of rotating a phonograph record about the record's central axis with an accurately defined speed, in such a way that substantially no variations occur in angular velocity, and no movements are introduced perpendicular to the plane of the record. It therefore follows that the turntable itself should rotate uniformly about its own central axis and thus provide uniform movement of each point of the turntable at which any contribution might be made to the production of undesired signals in the pick-up element.
Bearing arrangements for turntables must clearly fulfill two functions. First, a turntable mounted with its rotational axis vertical requires a thrust bearing function to support the weight of the turntable, any records on it, and any other downward forces due to mechanisms or vertical force exerted by a pick-up or other sensor. Secondly, radial bearing functions must position the turntable in the horizontal plane and maintain the verticality of the turntable axis. To maintain verticality it is then necessary to have effectively two radial bearings or portions thereof separated by an axial distance, such bearings being commonly referred to as the upper bearing and the lower bearing.
The basic type of radial lower bearing is a cylindrical bearing. However, such bearings normally exhibit play, and because of the slight eccentricity of the pivot relative to the central line of the spindle, which always exists in practice, the cylindrical pivot journal thus causes the spindle to move transversely relative to the bore of the bearing, within the limitations of the play. This gives rise to varying moments of friction at the circumference of the spindle, and therefore may cause load variations which in turn cause speed variations of the turntable. Such variations contribute to an undesired noise in the audio system, generally denoted as "wow and flutter," and which may be expressed numerically as a wow and flutter value.
Similarly, the use of a simple cylindrical upper bearing for the upper radial bearing can cause transverse movements of the turntable and variations of the angular velocity of the spindle relative to the bearing. The spindle will not constantly contact the upper bearing at the same location, but at a location that depends on the co-efficient of friction and the prevailing transverse force. For each revolution of the spindle instantaneous variations of the co-efficient of friction occur because the spindle does not have the same friction properties over its entire surface. Further, variations in the transverse forces which act locally on the spindle are caused by variations in the friction between the pick-up element and the record, by a record surface cleaner if one is used, and by any variations in the transverse force which may be exerted by the turntable drive. As with the lower bearing, these effects also contribute to wow and flutter.
2. Description of the Prior Art
U.S. Pat. No. 3,082,635 describes a record player having an improved turntable bearing arrangement which reduces undesired vibration of the turntable and resulting noise in the ultimately re-produced sound. In this known record player a lower combination thrust and radial bearing is used, plus a sleeve bearing nearer the turntable. The lower bearing comprises a bearing element having a conical cavity, engaged by a ball-shaped end of the turntable spindle. This engagement provides a substantially circular contact surface between the bearing element and the spindle end. In comparison with arrangements having a plain radial lower bearing such as a cylindrical sleeve bearing, this known player provides a reduction of the periodically changing frictional forces which would otherwise act at the circumference of the spindle.
The upper bearing is a sleeve bearing near the underside of the turntable, having an angular bearing element made of polytetrafluoroethylene plastic, which cooperates without play with a locally cylindrical portion of the spindle. Freedom from play is obtained by slight deformation of the plastic bearing element. However, such deformation increases the friction torque, while in the event of wear of the bearing surfaces play may still arise between the spindle and the bearing.